Emission spectroscopy based on plasma sources is a well accepted approach to elemental analysis. It is desired that an electrical plasma suitable as an emission source for atomic spectroscopy of a sample should satisfy a number of criteria. The plasma should produce desolvation, volatilization, atomization and excitation of the sample. However the introduction of the sample to the plasma should not destabilize the plasma or cause it to extinguish.
One known and accepted plasma source for emission spectroscopy is a radio frequency (RF) inductively coupled plasma (ICP) source, typically operating at either 27 MHz or 40 MHz. In general, with an RF ICP source the plasma is confined to a cylindrical region, with a somewhat cooler central core. Such a plasma is referred to as a “toroidal” plasma. To perform spectroscopy of a sample with an RF ICP source, a sample in the form of an aerosol laden gas stream may be directed coaxially into this central core of the toroidal plasma.
Although such plasma sources are known and work well, they generally require the use of argon as the plasma gas. However, argon can be somewhat expensive and is not obtainable easily, or at all, in some countries.
Accordingly, there has been ongoing interest for many years in a plasma source supported by microwave power (for example at 2.45 GHz where inexpensive magnetrons are available) which can use nitrogen, which is cheaper and more widely available than argon, as the plasma gas.
However, emission spectroscopy systems based on microwave plasma sources have generally shown significantly worse detection limits than systems which employ an ICP source, and have often been far more demanding in their sample introduction requirements.
For optimum analytical performance of the emission spectroscopy system, it is thought that the plasma should be confined to a toroidal region, mimicking the plasma generated by an RF ICP source.
It turns out to be much more difficult to produce such a toroidal plasma using microwave excitation than it is in for RF ICP source. With an RF ICP source, a current-carrying coil, wound along the long axis of a plasma torch, is used to power the plasma. The coil produces a magnetic field which is approximately axially oriented with respect to the long axis of the plasma torch, and this, in turn, induces circulating currents in the plasma, and these currents are symmetrical about the long axis of the plasma torch. Thus, the electromagnetic field distribution in the vicinity of the plasma torch has inherent circular symmetry about the long axis of the plasma torch. So it is comparatively easy to produce a toroidal plasma with an RF ICP source.
However, the waveguides used to deliver power to microwave plasmas do not have this type of circular symmetry, and so it is much more difficult to generate toroidal microwave plasmas.
There is therefore a desire to provide an improved microwave plasma source which can offer performance which approaches that of RF ICP, together with characteristics such as small size, simplicity and relatively low operating costs.